衍射声时技术的测量原理是以测出缺陷的（）为基础的。
参考答案：端头位置
In this school, ______ students work very hard, ______ they are very happy.
A．although, /
B．although, but
C．/, although
D．but, although
设
若x=rcosθ，r=rsinθ，则I在极坐标(r，θ)中的累次积分为______．
若x=rcosθ，r=rsinθ，则I在极坐标(r，θ)中的累次积分为______．